Charged-particle-beam microscopy is useful for measuring nanoscale features on a specimen. For accurate measurement, the charged particle beam should impact the specimen perpendicular to the sample surface. If the feature to be measured is tilted with respect to the beam, the measurement will be inaccurate. Aligning the sample crystal structure with the beam is referred to as “zone axis alignment.” A “zone axis” is a major axis of symmetry of the crystal.
Because of the wave nature of electrons, electrons that pass through a crystalline sample interfere with each other, reinforcing the electron beam in some regions and cancelling the beam in other regions. The interference forms a diffraction pattern on the back focal plane of an objective lens positioned below the sample. The diffraction pattern consists of a pattern of bright spots on a darker background. Each bright region represents a peak in the electron signal caused by diffraction from a specific set of planes within the crystal structure. The position of the spots in the diffraction pattern can therefore be used to identify the type of material in the sample. Identifying the set of crystal planes corresponding to a particular spot is referred to as “indexing” the spot. Indexing can be performed using geometrical relationships of the crystal planes and the microscope geometry.
FIG. 1 shows an example of an electron beam diffraction pattern 102 composed of diffraction spots 104. The symmetry of the pattern indicates that the electron beam is parallel to a zone axis of the crystal. Tilting the specimen, so that the beam is no longer parallel to the zone axis, has a pronounced effect on the diffraction pattern. A tilted specimen produces a pattern that appears as a series of diffraction spots arranged in an arc, known as a Laue circle. FIG. 2 shows a sample diffraction pattern 202 from a sample tilted with respect to the electron beam, with the diffraction spots 204 forming an arc. From the diffraction pattern, it is possible to determine the tilt of the zone axis of the sample relative to the electron beam. Once the tilt is determined, the sample can be reoriented to align it with the beam.
One method of using a diffraction pattern to align a sample with a beam is described by Jansen et al, “Towards automatic alignment of a crystalline sample in an electron microscope along a zone axis,” Ultramicroscopy 125(2013) 59-65. The degree and direction of specimen misalignment is determined by fitting a circle 302 to the arc of diffraction points 304 as shown in FIG. 3. The direction and magnitude of the radius vector 306 between the center 308 of the fitted circle and the transmitted beam spot of the diffraction pattern arc correspond to the direction and magnitude of specimen misalignment. The radius vector is converted to a stage tilt to reorient the zone axis to be parallel to the beam as indicated by the diffraction pattern of FIG. 4 acquired after correction. The method may be iterated until desired alignment is achieved. Once the sample is aligned, features on the sample can be accurately measured.
This method is based on heuristics, and is relatively slow. It can be difficult or impossible to fit a circle to some diffraction patterns. For example, in the pattern of FIG. 5, it can be difficult to determine which points to include in the circle fitting. In the image shown in FIG. 6, taken from a sample in which the beam is oriented far from the sample zone axis, not many spots are visible, and the spots that are visible do not appear to form a circle at all. In addition, the pattern of FIG. 6 appears to suffer from ‘two beam’ dynamic diffraction. The classical diffraction model assumes the particle (X-ray or electron) scatters only once during its interaction with the specimen. Electrons strongly interact with matter, so multiple scatter events occur. The beam scatters, then the scattered beam scatters again, etc. This phenomenon is known as dynamic diffraction. Thicker material causes more scattering. For silicon having a thickness of less than about 20 nm, dynamic diffraction can be safely ignored. Dynamic diffraction can cause the Laue circle method to fail. In addition, the method is not useful if the sample misalignment is less than 1 degree, because the diffraction pattern is not sufficiently circular for matching. Moreover, it can be difficult to determine the accuracy of the tilt measurement from circle-fitting routines.
There are several other ways of determining zone axis tilt using diffraction patterns. For example, using the Weiss zone law, in which the pattern is indexed and two-three prominent spots are selected to find the zone axis. This method is only accurate to about 2-3 degrees at best, which is inadequate for crystal alignment.
Using Kikuchi lines is a very accurate, standard technique for determining zone axis tilt. It can be done manually and/or with computer aid. Kikuchi lines result from dynamic diffraction effects and are only readily visible with thick and/or high Z (atomic number) samples. The Kikuchi line technique is therefore not possible for use with thin samples, such as samples utilized in electronics industry where node sizes are now approaching 10 nm.
Another method entails indexing a series of diffraction patterns and computing the crystal unit cell parameters and orientation using least squares methods. This method requires more than one diffraction pattern and involves tilting the stage, which causes translation in XYZ. This is highly undesirable for aligning the crystal orientation in an electron beam system because all alignments are local since the specimen undulates like a piece of crumpled paper.
U.S. Pat. Pub 2011/0220796 to Nicolopoulos et al. describes a method for electron diffraction tomography of a crystal sample. A polycrystalline material is imaged by scanning the beam in raster. At each beam position diffraction patterns are obtained by precessing the beam like a top to suppress dynamic diffraction effects. The diffraction patterns are compared to reference patterns to determine the crystal orientation is then computed. A map is generated showing crystal orientation as a function of beam position. Grains in a material are readily visible by examining such a texture map. The determination of the orientation is not sufficiently accurate for aligning a sample for metrology. This technique also appears to require indexing the diffraction patterns and maintaining a library for every material in question.